In the course of their work in the field of piezoelectric pumps, the applicants have recognized that there is a need for means to coat piezoelectric substrates with a semi-conducting material which is susceptible to doping or other techniques for the purpose of imparting electrically-conducting properties to the coating. Such substrates are particularly suitable for use in ink-jet printing devices of the type described in applicants' International Patent Application No. PCT/EP90/02119, filed on Dec. 6, 1990, in which a piezoceramic wafer having planar opposite first and second surfaces has grooves or channels extending inwardly therein. The groove or grooves of the first planar surface lie offset to the groove or grooves of the second planar surface with the grooves partially overlapping in the direction of their depth. Each groove of one of these surfaces is intended to function as a droplet ejector channel. This is accomplished by providing defined areas of the piezoceramic wafer with a conductive coating, thereby forming electrodes for each ejector channel. Thereafter, the material can be polarized by applying an electrical voltage between the electrodes. An electrical pulse is subsequently applied across the electrodes, causing a rapid decrease in the volume of the ejector channel, thereby effecting discharge of the liquid from within the channel. Discharge of the liquid in a longitudinal direction is assured by covering the open longitudinal extension of the ejector channels with a cover plate. This cover plate also serves to impart stability to the piezoceramic wafer.
Even though the above-described type of ink-jet printing device theoretically offers a much more versatile printing system, its usefulness has, at least up to now, been hampered by the difficulty in obtaining a satisfactory bond between the cover plate and the coated piezoceramic wafer. This is because satisfactory bonding is very much dependent on the smoothness of the surfaces which are to be bonded. It is relatively easy to obtain an adequately smooth surface on the cover plate since in most cases the cover plate is made from glass. Difficulties arise, however, with the coated piezoceramic wafer. A satisfactory degree of smoothness can be imparted on the piezoceramic wafer itself, and this must then be maintained after application of the electrode coating. Thus, the coating process must result in a piezoceramic wafer having an electrode coating with a sufficiently smooth surface.
When the applicants have performed electrode application methods which result in a crystalline coating of semi-conducting material, they have been found to be too uneven to be bonded by, for example, anodic bonding, unless subjected to subsequent smoothing operations. Attempts have also been made to bond a crystalline silicium-coated piezoceramic wafer obtained by chemical vapor deposition (CVD) to glass by distributing adhesive over the glass cover plate by means of centrifugal spinning. In order to avoid the need for an adhesive agent, attempts have been made to coat piezoceramic wafers by means of sputtering. While sputtering results in a relatively smooth coating, sputtering techniques cannot provide an ejector channel for an ink-jet printing device whose side walls are adequately coated with electrode material.
A CVD method for producing either an undoped or a doped amorphous silicium coating on unspecified substrates is disclosed in EP-A-0 143 701. In this method, the coating is achieved by thermally decomposing disilane within a heated quartz chamber containing the substrates. This method reputedly offers the advantage that disilane is obtained within the chamber by a chemical reaction between silane and atomic hydrogen.
It is therefore an object of the present invention to provide a method of coating a piezoelectric substrate, and preferably a piezoceramic substrate, and preferably in the form of a wafer, with a substantially uniform layer of a semi-conducting material, which resulting layer is sufficiently smooth to allow bonding between the thus coated piezoelectric substrate and a cover plate.